1. Field of the Invention
The present teaching relate to a method for determining the catalyst lifetime for catalysts producing single walled carbon nanotubes, (hereinafter “SWNTs”). The method is based on analysis of Raman spectra of nanotubes obtained by sequential introduction of isotopically-labeled reaction components, such as, gases 12CH4 and 13CH4, at different stages of catalyst activity during nanotube production. The growth of the nanotubes can be evaluated by mass spectrometry.
The present teachings also relate to the modification of catalyst composition, for example, using Fe/Mo instead of Fe, that can increase the lifetime approximately three times and thereby increasing the yield of the SWNT. An increase of 100° C. in the synthesis temperature shortened the lifetime from, for example, about 35 minutes to about 10 minutes, however the growth rate increased resulting in a similar overall yield of SWNT. No evidence of lifetime changes was found when the carbon feedstock varied on the order of 8 times, but instead resulted in variation of the amount of disorder carbon formed.
2. Discussion of the Related Art
Catalytic chemical vapor deposition (CCVD) is considered the most controllable and versatile method for the growth of carbon nanotubes. Along with the traditional iron-based family of catalysts and co-catalysts for the growth of SWNT, new families of noble metal-based catalysts (Pt, Pd, Au, Ag), together with Cu, have been reported. This brings more diversity and complexity to the elucidation of the catalyst features favorable for nanotube growth, since catalyst properties and parameters like preparation method, pretreatment, diameter and their crystallographic and electronic structure have a remarkable influence on nanotube growth.
Among catalyst properties, the lifetime is a key characteristic, especially for growing long tubes needed for particular applications. Previously reported approaches for estimation of the catalyst lifetime are mainly based on post synthesis measurements of the nanotubes' length with time.
Methods include monitoring in-situ height during growth of the SWNT, and inducing growth marks during growth of multi-walled carbon nanotube (hereinafter “MWCNT”) arrays.
All of the above reported approaches are generally applicable for individual tubes or very well aligned forests of tubes, where the length of the nanotube can be measured. A need exists for a simple method that allows one to evaluate the period of catalyst activity and determine the duration favorable for the growth of SWNTs. Of particular interest are methods for studying and determining the catalyst lifetime for bulk growth of SWNTs. Additionally, a need exists for understanding the dependence of the catalyst lifetime on, such factors as, for example, catalyst composition and different nanotube synthesis parameters.